GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1997 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Backfire

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Definition: Fuel ignites in the intake manifold, or in the exhaust system, making loud popping noise.

Preliminary Checks
    • Refer to Important Preliminary Checks .
    • Refer to Visual/Physical Checks .
    • Search for bulletins.

Sensor/System
    • Check for the following intermittent ignition system conditions:
       - Intermittent CKP Sync or CKP 18X Signal.
       - Intermittent ignition feed circuit or sensor ground circuit to the crankshaft position sensor or the camshaft position sensor.
       - Intermittent 3X reference signal.
       - Intermittent camshaft position signal
    • Check the Mass Air Flow sensor conditions. Ensure that the MAF Sensor honeycomb is not damaged.

Fuel System
    • Check for incorrect fuel pressure. Refer to Fuel System Pressure Test .
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Test .
    • Ensure each injector harness is connected to the correct injector/cylinder. Refer to Engine Controls Schematics Wiring Diagrams.

Ignition System
    • Check for proper ignition voltage output using the following steps:
       - Clip the spark tester J 26792 to engine ground.
       - Connect one end of a spark plug wire to the spark tester; leave the other end of the spark plug wire to the coil being tested.
       - Connect one end of another spark plug wire to the other coil tower; connect the other end of the spark plug wire to ground.
       - Crank the engine while observing the spark tester; a crisp blue spark should be observed.
       - Repeat the above steps for each coil.
    • If adequate spark is not present at the coil(s), check for the following conditions:
       - Coil(s) - Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

Coil Resistance
5000 - 8000 ohms (5K - 8K ohms)

       - Spark Plug wires - Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Resistance
Less than 10000 ohms (10K ohms)

   Important: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. Ignition components will arc to ground when a secondary component is faulty.

       - Defective module.
       - Ignition System Wiring - Loose ignition module feed or ground connection, or damaged system wiring.
    • Remove spark plugs and check for the following conditions:
       - Fouled plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical
    • Check engine mechanical:
       - Check compression.
       - Sticking or leaking valves.
       - Worn camshaft lobe(s).
       - Valve timing.
       - Bent push rods.
       - Worn rocker arms.
       - Broken valve springs.
       - Excessive oil in combustion chamber-Leaking valve seals.
       - Low cylinder compression.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check Intake and exhaust manifold(s) for casting flash. Refer to Engine Mechanical .
    • Check TCC operation. Refer to Automatic Transmission Diagnosis .
    • Check the exhaust system for a possible restriction.
       - Inspect exhaust system for damaged or collapsed pipes.
       - Inspect mufflers for heat distress or possible internal failure.
       - For possible plugged Three-Way catalytic converter. Refer to Restricted Exhaust System Check and Exhaust System .